The Hidden (Animorphs #39)
The Yeerks have discovered and repaired a damaged Helmacron ship. They know of its morph-seeking capabilities, and they plan to use the ship to capture the "Andalite bandits" and to find Elfangor's blue cube. The one that gave the kids the ability to morph.
Cassie, the other Animorphs, and Ax are in a pretty bad situation because they can't leave the cube in one place, they can't morph without being discovered, and they have to keep moving. It looks like this may be a battle the Visser can't lose...